The concept of hot-plugging electrical equipment refers to the ability to insert or remove equipment relative to a base unit without having to turn the system power off or reboot the system. Hot-plugging has its roots in the telephone switch room and is fairly well established. However, hot-plugging is relatively new to the personal computer equipment domain. One of the first systems to support hot-plug peripheral component interconnect (PCI) adapters in a server isolated the entire bus at the bus bridge controller and could resolve to the bus level. A subsequent system improved on this concept to support slot-wise granularity. The user could insert or remove single adapters while maintaining other operations on the same or peer PCI bus.
There are several problems with these designs. The PCI bus specification describes certain electrical signaling characteristics which must be met in order for the bus to function properly. One such parameter is associated with the trace or wire length of the bus signals. It is important to maintain consistency among the lengths of certain groups of bus signals to control the arrival time of the signals. Another problem concerns the package size and pin count. The best package size and pin count would be a single pin package that was no larger than the trace width. Unfortunately, traces are much narrower than any package would allow, and assembly would not be facilitated by such a package even if it existed.
One solution for hot-pluggable PCI adapter cards would utilize a field effect transistor (FET) in series with the PCI signal. FETs are typically available in surface mount packaging with a 10 or 20-bit width. Unfortunately, using this type of packaging increases the wiring length required to connect the FET to the PCI connector and the system bus. Although single-bit FETs are available, the aggregate space consumed by single-bit packages consumes too much space on the system board. Furthermore, adding the FET packages in close proximity to the PCI connector causes difficulty in manufacturing and rework of the system board.